Junction functions are similar to BTE(business Transaction events) where in which we can place our logic (logic can be some validations or triggering some idocs etc.,)
The function module used to find the existence of junction functions is
RWIN_CHECK_SUBSET
and for BTE's you will finding the couple of tables for customer enhancement as TBE31,TBE34 etc.,
and in the same way for junction functions you have TRWPR table for this purpose
where you can add your own Z function module and include your validation

This Junction functions are used in recent developments such as IS-OIL developments and mostly it is used for Posting process such as MIRO,FBV2,FV60 etc.,